Japanese knotweed are two words no homebuyer or seller wants to hear. But there are ways to deal with the problem and even secure mortgage finance. Find out more with our Q&A.

Q. What is Japanese knotweed?

A. Japanese knotweed, the common name for Fallopia japonica, is a highly invasive plant which thrives around water sources such as ponds, canals and lakes, as well as along railway embankments and in large open spaces.

Growing vigorously from year-to-year (‘perennial’), it produces tall canes up to 3m (10ft) high during summer. 

Q. Where did it come from?

Native to Japan, China and Korea, Japanese knotweed was introduced to the UK as an ornamental garden plant in the mid-19th century. It has since spread rapidly across the country. 

Q. What does it look like?

A. Its buds, which start sprouting from the base of the plant between February and March, are red. These develop into shoots which are also reddish in appearance. Shoots grow into hollow canes (stems) which look a bit like bamboo but are green in colour and have characteristic purple flecks.

Its leaves are shovel-shaped and up to 14cm in length. They appear yellowish-brown in spring, becoming darker – a lush green colour – during summer.  In autumn, Japanese knotweed produces clusters of small, creamy-white flowers.Japanese Knotweed in a garden

Q. Why is Japanese knotweed a problem?

A. Japanese knotweed can cause structural damage to homes and other buildings. It pushes through cavity walls and drains, and up through asphalt, cracks in concrete and driveways. It can also cause significant delays and costs to building development.

When the plant invades natural habitats (often through fly-tipping), it out-competes and smothers other plants and animals that usually live there. It clogs up routes used by wildlife and takes up precious space along rivers and around hedges and roadsides.

Once it takes hold, Japanese knotweed is extremely difficult to eradicate.

Q. Where is it mostly found?

A. Japanese knotweed is found all over the UK. But, according to removal specialist Environet UK, it's particularly prevalent in south Wales. See its heat map below:Japanese Knotweed heat map

Q. How common is it?

A. Environet UK estimates that between 1% and 5% of homes in this country are affected by Japanese knotweed.

Q. How does it spread?

A. Japanese knotweed spreads in several ways, which is what makes it so difficult to wipe out.

Firstly, it develops an extensive network of underground stems called ‘rhizomes’ and the plant can sprout from very small sections of these. Trials have shown that as little as 0.7 grams of rhizome – about 10mm, or the length of your fingernail – can produce a whole new plant within 10 days.

Freshly-cut stems, if they become buried in soil or immersed in water, also quickly produce more shoots. This is why the use of simple garden strimmers can help spread the plant.

Japanese knotweed is also a great survivor. Even when dried or composted, its crown will produce new canes if it comes into contact with soil or water.Japanese Knotweed in the sun

Q. Why hasn’t it been eradicated?

A. For a start, people may not be aware that it is growing on nearby public land – or even on their own property.

When they are aware, householders might try to kill the plant with chemical sprays – weedkiller – and assume they have eradicated it after just one treatment. But, according to the Environment Agency (EA), rhizomes can remain dormant in the ground for as long as 20 years, meaning there's every chance of it returning if further treatment is not carried out.Japanese Knotweed in flower

Q. What is the Government doing about the problem?

A. The plant was included in the Wildlife and Countryside Act 1981, making it an offence to, "plant or otherwise cause Japanese Knotweed to grow in the wild.

The Government also amended the Anti-social Behaviour, Crime and Policing Act 2014 to include Japanese knotweed. This means it's now possible to get an ASBO if you allow the plant to spread from your land onto your neighbours'.Man eradicating Japanese Knotweed

Q. So, will I get into trouble if it is growing on my property?

A. If you don't take the necessary action, yes. You could be fined up to £5,000 or be sent to prison for up to two years if you allow contaminated soil or plant material from any waste you transfer into the wild.

However, the Royal Institution of Chartered Surveyors (RICS) says that the most active area of legal action is civil litigation, where one party seeks financial damages from another. This is because knotweed can decrease property property prices by up to 20%.

Did you know? Network Rail was recently sued over a long-established infestation of Japanese knotweed on a rail embankment, which devalued adjoining properties in a Welsh town. The presence of the plant was found to be an ‘actionable nuisance’. The judge ruled that Network Rail should have done more to eradicate the knotweed near the claimants’ properties.

Q. Do I need to tell the authorities if I discover it?

A. It's not illegal to have Japanese knotweed in your garden. But, it is your responsibility control it and prevent it from becoming a problem in your neighbourhood.

You may need to contact someone officially when it comes to getting rid of it too. For example:

  • When using chemicals to kill the plant. You’ll need to get permission from the EA if the plants are near water. You might also need an environment permit/registered waste exemption/trade effluent consent.
  • When burying it as plant waste. You won’t normally be allowed to bury waste on land unless it’s at a landfill site that’s got a suitable permit, so check with the EA first.

Japanese Knotweed sign

Q. How do I get rid of it?

A. First of all, double-check it is actually Japanese knotweed, as it can often be confused with other plants. A qualified surveyor could help here, or a company specialising in the vegetation’s removal.

Once confirmed, options for eradication include long-term and consistent use of weedkiller (most likely a Glyphosate based product). The chemical treatment will need to be carried out over a number of years.

It can also be buried (check for necessary permissions); or dug up and physically removed – particularly the rhizome.

Some companies also offer a vertical root barrier treatment to protect against spread. Continued monitoring is also strongly advised.

Finally, a plant sucker is being trialled as a possible form of natural, biological control at a handful of sites in the UK, but it is not available to gardeners – yet.

Treatment costs can run into several thousand pounds, but it all depends on how severe the problem is and how quickly it is tackled.

Among the many websites offering advice, it’s worthwhile looking at Cornwall Council’s useful links. And professional removal experts and national bodies include:

But small-scale occurrences of the plant in domestic gardens may not always require professional attention to be effectively controlled, according to recent research from the University of Exeter.

Bee on Japanese Knotweed

Q. I’m thinking about buying/selling a house, but I’m worried about Japanese knotweed. What should I do?

A. It pays to get a professional survey done. Environet UK recommends contacting a reputable Japanese knotweed removal specialist who will conduct a survey of the affected area.

The cost of surveys start from around £240 plus VAT.

Make sure the knotweed specialist provides full evidence of their treatment plan, together with an insurance-backed guarantee that covers the work (and any potential further treatment) for a minimum of five years, preferably 10.

The RICS also has links to professional surveyors with experience in this area, who will give comprehensive inspections – including along and over boundaries.

The best time to survey Japanese knotweed is during the spring and summer months (May to October) when the plant is actively growing. However, an expert should be able to spot it all year round.

Japanese Knotweed leaves

Q. Can I get a mortgage on a property with Japanese knotweed?

A. So long as a qualified surveyor has identified the knotweed, provided a report and a Japanese knotweed management plan (which includes an accurate record of the infestation and how it is to be eradicated) is in place, it is possible to get a mortgage.

David Hollingworth, at broker L&C Mortgages, says: “Lenders have evolved their policies, so it's not as black and white as it used to be.”

However, he recommended that if you are planning on selling, you may want to deal with the problem before putting your home on the market: "If you don't, you risk the buyer trying to knock the price down, and the treatment process isn’t cheap”.

Mark Harris, chief executive of mortgage broker SPF Private Clients, adds: “Most lenders are happy to lend on a property where Japanese knotweed has been found in the past, so long as there has been a course of treatment and a certain amount of time has lapsed. Much depends on the valuer’s comments and proximity of the knotweed to the property.”Japanese Knotweed growing through a property

Q. I know I have Japanese knotweed on my property. Do I have to tell the buyer?

A. In a word, yes. Environet UK explains that, although it’s not a notifiable weed, when you come to sell a property, you are required to answer a set of pre-contract enquiries. The Law Society’s TA6 form asks sellers: "is the property affected by Japanese knotweed?’ to which you can answer, ‘yes’, ‘no’ or ‘not known’.

If your buyer feels they have been the victim of misrepresentation, they can sue you under the Misrepresentation Act.

Links to further information:

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